1. Field of the Invention
The present invention relates to a telescopic lens system, and in particular, relates to a telescopic lens system applied to a video camera or a digital camera and the like.
2. Description of the Prior Art
As an optical system used as a telescopic lens system, a Petzval-type optical system, including a positive first lens group constituted by a combination of a positive lens element and a negative lens element, and a positive second lens group also constituted by a combination of a positive lens element and a negative lens element, is known in the art.
In this Petzval-type optical system, in each of the first and second lens groups, by providing a positive lens element with a strong converging function and a negative lens element with a strong diverging function, spherical aberration and chromatic aberration can be cancelled out, so that optimum optical performance can be obtained.
However, in a conventional Petzval-type optical system, it is necessary to correctly align the optical axes of the positive and negative lens elements of the first and second lens groups. This is because if the two optical axes thereof are even slightly out of alignment, aberrations, such as axial coma, etc., largely occur. In particular, in video cameras and digital cameras, miniaturization of the image pick-up device has further increased, and higher definition thereof is being achieved. Therefore higher optical performance is required to the optical system, so that higher precision is necessary for the manufacturing process of video and digital cameras. As a result, the cost thereof increases.
The present invention provides a Petzval-type telescopic lens system which has (i) an f-number of about 4, and (ii) a half angle-of-view of about 4 degrees; and which (iii) is constituted by about four lens elements, (iv) is free from optical deterioration caused by decentration of lens elements, (v) is produced at a low cost, and (vi) has optimum optical performance.
The present invention is devised based on the conception that by cementing the positive lens element and the negative lens element of the first lens group in a Petzval-type telescopic lens system, the optical axes of the positive and negative lens elements are prevented from being out of alignment, and the correcting of spherical aberration, which is considered to be important to attain preferable optical performance, is intensively performed by the first lens group, so that occurrences of aberrations can be reduced.
According to an aspect of the present invention, there is provided a telescopic lens system including a positive first lens group, and a positive second lens group, in this order from the object. The first lens group includes cemented lens elements having a positive lens element and a negative lens element. The second lens group includes a positive lens element and a negative lens element. The telescopic lens system satisfies the following condition:
0.6 less than xcexa3|S1I|/xcexa3|S1|xe2x80x83xe2x80x83(1) 
wherein
xcexa3|S1I| designates the total sum of the absolute values of the spherical aberration coefficients (Seidel coefficients) of the first lens group, and
xcexa3|S1| designates the total sum of the absolute values of the spherical aberration coefficients (Seidel coefficients) of the entire telescopic lens system.
The telescopic lens system according to the present invention preferably satisfies the following conditions:
0 less than xcexa3S1/xcexa3S1I less than 3.5xe2x80x83xe2x80x83(2) 
20 less than xcexdIpxe2x88x92xcexdInxe2x80x83xe2x80x83(3) 
wherein
xcexa3S1 designates the total sum of the spherical aberration coefficients (Seidel coefficients) of the entire telescopic lens system;
xcexa3S1I designates the total sum of the spherical aberration coefficients (Seidel coefficients) of said positive first lens group;
xcexdIp designates the Abbe number of the positive lens element of the positive first lens group, and
xcexdIn designates the Abbe number of the negative lens element of the positive first lens group.
The telescopic lens system according to the present invention preferably satisfies the following condition:
xe2x88x921 less than xcexa3PII/xcexa3P less than 0xe2x80x83xe2x80x83(4) 
wherein
xcexa3PII designates the Petzval Sum of the positive second lens group; and
xcexa3P designates the Petzval Sum of the entire telescopic lens system.
The telescopic lens system according to the present invention can satisfy the following condition:
xe2x88x920.2 less than xcexa3S2/xcexa3S2II less than 0xe2x80x83xe2x80x83(5) 
wherein
xcexa3S2 designates the total sum of the coma coefficients (Seidel coefficients) of the entire telescopic lens system; and
xcexa3S2II designates the total sum of the coma coefficients (Seidel coefficients) of the positive second lens group.
The telescopic lens system according to the present invention preferably satisfies the following conditions:
0.5 less than f/fI less than 0.9xe2x80x83xe2x80x83(6) 
0.15 less than DI-II/f less than 0.7xe2x80x83xe2x80x83(7) 
wherein
f designates the focal length of the entire telescopic lens system;
fI designates the focal length of the positive first lens group, and
DI-II designates the distance between the positive first lens group and the positive second lens group.
The telescopic lens system according to the present invention can satisfy the following condition:
1 less than ACI/ACIIxe2x80x83xe2x80x83(8) 
wherein
ACI designates the total sum of the absolute values of the reciprocal of the product of the focal length and the Abbe number (|1/(fi*xcexdi)|) of each lens elements in the positive first lens group; and
ACII designates the total sum of the absolute values of the reciprocal of the product of the focal lengths and the Abbe number(|1/(fi*xcexdi)|) of each o lens elements in the positive second lens group.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-300479 (filed on Sep. 28, 2001) which is expressly incorporated herein in its entirety.